U.S. patent number 8,273,277 [Application Number 12/440,968] was granted by the patent office on 2012-09-25 for process for producing a proton-conducting, polyazole-containing membrane.
This patent grant is currently assigned to BASF Fuel Cell GmbH. Invention is credited to Gunter Christ, Christian Dorr, Tequila Harris, Oemer Uensal, Daniel Walczyk, Mathias Weber.
United States Patent |
8,273,277 |
Uensal , et al. |
September 25, 2012 |
Process for producing a proton-conducting, polyazole-containing
membrane
Abstract
A method for producing a proton-conducting, polyazole-containing
membrane, in which A) a composition containing polyphosphoric acid
and at least one polyazole and exhibiting a solution viscosity in
the range from 10 Pas to 1000 Pas, measured to DIN 53018 at the
temperature at the orifice during production of the membrane, is
pressed through an orifice at a temperature in the range from
25.degree. C. to 300.degree. C., and B) the composition is then
solidified.
Inventors: |
Uensal; Oemer (Mainz,
DE), Weber; Mathias (Russelsheim, DE),
Christ; Gunter (Hunstetten, DE), Dorr; Christian
(Eppstein, DE), Walczyk; Daniel (Troy, NY),
Harris; Tequila (Troy, NY) |
Assignee: |
BASF Fuel Cell GmbH
(DE)
|
Family
ID: |
38982455 |
Appl.
No.: |
12/440,968 |
Filed: |
September 11, 2007 |
PCT
Filed: |
September 11, 2007 |
PCT No.: |
PCT/EP2007/007896 |
371(c)(1),(2),(4) Date: |
October 30, 2009 |
PCT
Pub. No.: |
WO2008/031554 |
PCT
Pub. Date: |
March 20, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100181697 A1 |
Jul 22, 2010 |
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Foreign Application Priority Data
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Sep 12, 2006 [DE] |
|
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10 2006 042 760 |
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Current U.S.
Class: |
264/176.1 |
Current CPC
Class: |
H01M
8/1048 (20130101); H01M 8/1088 (20130101); B01D
71/62 (20130101); B01D 71/82 (20130101); C08G
73/18 (20130101); C08G 73/22 (20130101); H01M
8/1072 (20130101); B01D 67/002 (20130101); C08J
5/2256 (20130101); H01M 8/103 (20130101); H01M
8/1041 (20130101); H01M 8/1081 (20130101); Y02P
70/50 (20151101); B01D 2323/42 (20130101); Y02E
60/50 (20130101); B01D 2323/12 (20130101); B01D
2323/08 (20130101); B01D 2323/06 (20130101); C08J
2379/06 (20130101); H01M 2300/0082 (20130101) |
Current International
Class: |
B28B
3/20 (20060101) |
Field of
Search: |
;264/176.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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19527435 |
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Jan 1997 |
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DE |
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19959289 |
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Jun 2001 |
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DE |
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10213540 |
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Feb 2004 |
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DE |
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10246459 |
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Apr 2004 |
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DE |
|
10246461 |
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Apr 2004 |
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DE |
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10144815 |
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Jan 2005 |
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DE |
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1657274 |
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May 2006 |
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EP |
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WO-96/01177 |
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Jan 1996 |
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WO |
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WO-96/13872 |
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May 1996 |
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WO |
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WO-2004034499 |
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Apr 2004 |
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WO |
|
Primary Examiner: Thrower; Larry
Attorney, Agent or Firm: Connolly Bove Lodge & Hutz
LLP
Claims
The invention claimed is:
1. A method for producing a proton-conducting, polyazole-containing
membrane which comprises A) extruding a composition containing 70
wt. % to 99.5 wt. % of polyphosphoric acid and 0.5 wt, % to 30 wt.
% of at least one polyazole through a slot, said composition and
exhibiting a solution viscosity in the range from 10 Pas to 1000
Pas, measured to DIN 53018 at the temperature at the slot, said
temperature being in the range from 120.degree. C. to 300.degree.
C. during production of the membrane, and B) solidifying the
composition through cooling and/or hydrolysis and/or
cross-linking.
2. The method according to claim 1, wherein the composition is
pressed through the orifice at a temperature in the range from
120.degree. C. to 180.degree. C.
3. The method according to claim 1, wherein the composition is used
which exhibits a solution viscosity in the range from 20 Pas to 800
Pas, measured to DIN 53018 at the temperature at the orifice during
production of the membrane.
4. The method according to claim 1, wherein the composition is used
which contains at least one polyazole with an intrinsic viscosity,
measured in at least 96 wt. % sulfuric acid, in the range from 0.3
to 10 dl/g.
5. The method according to claim 1, wherein the composition is
pressed through the orifice which, in each case relative to the
total weight thereof, contains 95.0 wt. % to 99.5 wt. % of
polyphosphoric acid and 0.5 wt. % to 5.0 wt. % of polyazole.
6. The method according to claim 1, wherein the slot has a slot
width in the range from 20 cm to 50 cm.
7. The method according to claim 1, wherein the slot has a slot gap
in the range from 800 .mu.m to 1600 .mu.m.
8. The method according to claim 1, wherein the slot has a ratio of
slot width to slot gap in the range from 100:1 to 1000:1.
9. The method according to claim 1, wherein the composition is
pressed through the slot with a pressure of at least 0.1 bar.
10. The method according to claim 5, wherein the polyphosphoric
acid and the polyazole are mixed in an extruder.
11. The method according to claim 1, wherein the polyazole is
produced in situ a) by reacting one or more aromatic and/or
heteroaromatic tetra-amino compounds with one or more aromatic
and/or heteroaromatic carboxylic acids or the derivatives thereof,
which contain at least two acid groups per carboxylic acid monomer,
or b) by reacting one or more aromatic and/or heteroaromatic
diaminocarboxylic acids.
12. The method according to claim 1, wherein the polyazole
comprises at least one phosphonic acid group.
13. The method according to claim 1, wherein the composition is
applied onto a support.
14. The method according to claim 13, wherein a plurality of
non-interconnected fields of the composition are applied onto the
support.
15. The method according to claim 1, wherein the composition is
solidified by treating it with moisture.
16. The method according to claim 1, wherein a composition is used
which contains at least one polymer which differs from polyazole.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
Related Applications
This application is a national stage application (under 35 U.S.C.
.sctn.371) of PCT/EP2007/007896, filed Sep. 11, 2007, which claims
benefit of German application 10 2006 042760.2, filed Sep. 12,
2006.
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a
proton-conducting, polyazole-containing membrane.
Polymer electrolyte membranes (PEM) are already known and, in
particular, are used in fuel cells. Sulfonic acid-modified
polymers, in particular perfluorinated polymers, are often used for
this purpose. One prominent example of these is Nafion.TM. from
DuPont de Nemours, Willmington USA. Proton conduction entails a
relatively high water content in the membrane, typically amounting
to 4-20 molecules of water per sulfonic acid group. Not only the
necessary water content, but also the stability of the polymer in
conjunction with acidic water and the reaction gases hydrogen and
oxygen, conventionally limit the operating temperature of the PEM
fuel cell stack to 80-100.degree. C. Under pressure, operating
temperatures can be raised to >120.degree. C. Otherwise, higher
operating temperatures cannot be achieved without a drop in fuel
cell performance.
However, for systems engineering reasons operating temperatures of
higher than 100.degree. C. in the fuel cell are desirable. The
activity of the noble metal-based catalysts present in the
membrane-electrode unit (MEU) is substantially better at elevated
operating temperatures. In particular when hydrocarbon "reformates"
are used, the reformer gas contains considerable quantities of
carbon monoxide which conventionally have to be removed by complex
gas preparation or purification. The tolerance of the catalysts to
CO contamination increases at elevated operating temperatures.
Furthermore, heat arises during fuel cell operation. However,
cooling these systems to below 80.degree. C. may be very expensive.
Depending on power output, the cooling devices may be of
substantially simpler design. That means that, in fuel cell systems
which are operated at temperatures of above 100.degree. C., the
waste heat is distinctly more readily utilisable and efficiency of
the fuel cell system can be increased by combined heat and power
generation.
Membranes with new conductivity mechanisms are generally used to
achieve these temperatures. One approach is to use membranes which
exhibit electrical conductivity without the use of water. The first
promising development in this direction is presented in publication
WO 96/13872. This in particular proposes using acid-doped
polybenzimidazole membranes which are produced by casting.
Documents DE 102 46 459 A1, DE 102 46 461 A1 and DE 102 13 540 A1
describe further developments of this type of membrane.
DE 102 46 461 A1 discloses proton-conducting polymer membranes
which are obtainable by a method which comprises the steps: A)
producing a mixture comprising polyphosphoric acid, at least one
polyazole (polymer A) and/or at least one or more compound(s)
which, on exposure to heat according to step B), is/are suitable
for forming polyazoles, B) heating the mixture obtainable according
to step A) under inert gas to temperatures of up to 400.degree. C.,
C) applying a layer using the mixture according to step A) and/or
B) onto a support, D) treating the membrane formed in step C) until
it is self-supporting, wherein at least one further polymer
(polymer B), which is not a polyazole, is added to the composition
obtainable according to step A) and/or step B), the weight ratio of
polyazole to polymer B being in the range from 0.1 to 50.
DE 102 46 459 A1 relates to proton-conducting polymer membranes
based on polyazoles containing phosphonic acid groups which are
obtained by a method which comprises the steps: A) mixing one or
more aromatic and/or heteroaromatic tetra-amino compounds with one
or more aromatic and/or heteroaromatic carboxylic acids or the
derivatives thereof which contain at least two acid groups per
carboxylic acid monomer, wherein at least a proportion of the
tetra-amino compounds and/or of the carboxylic acids comprises at
least one phosphonic acid group, or mixing one or more aromatic
and/or heteroaromatic diaminocarboxylic acids, at least a
proportion of which comprises phosphonic acid groups, in
polyphosphoric acid, to form a solution and/or dispersion, B)
heating the solution and/or dispersion obtainable according to step
A) under inert gas to temperatures of up to 350.degree. C. while
forming polyazole polymers, C) applying a layer using the mixture
according to step A) and/or B) onto a support, D) treating the
membrane formed in step C) until it is self-supporting.
DE 102 13 540 A1 relates to proton-conducting polymer membranes
based on polyvinylphosphonic acid which are obtainable by a method
which comprises the steps: A) dissolving a polymer, in particular a
polyazole, in phosphonic acid containing vinyl, B) forming a planar
structure using the solution according to step A) on a support C)
applying a starter solution onto the planar structure formed
according to step B) and D) polymerising the vinylphosphonic acid
present in the planar structure according to step C).
In these methods, it is intended for the planar structure to be
formed [step C) in DE 102 46 461 A1, step C) in DE 102 46 459 A1,
step B) in DE 102 13 540 A1] by means of per se known measures,
such as for example casting, spraying, knife coating, extrusion,
which are known from the prior art for polymer film production.
However, no further indications as to the exact procedure are to be
inferred from the documents.
Producing the above membranes by casting, spraying or knife coating
is unfortunately very complex and costly. It requires the use of
large quantities of solvent to dissolve and apply the polymer onto
the support, which solvent must subsequently be removed and
recovered. The method is furthermore very time-consuming and
permits only a low space-time yield. The fluctuations in quality
which are frequently to be observed between different production
batches constitute an additional problem. Furthermore, processing
polyazoles with comparatively high molecular weights is
particularly difficult due to the relatively poor solubility of
these polymers, the increasing non-uniformity of the corresponding
solutions and the increasing formation of bubbles.
Extruding the mixtures to form the corresponding planar structures
is also non-trivial. The problem in particular arises that, due to
the comparatively high temperatures, the polyazoles continue to
condense, so forming polymers with ever higher molecular weights,
whereby the properties of the polymers and the membranes, if they
can even be obtained, are significantly impaired. Furthermore, due
to the high molecular weight, processing of the polyazoles becomes
increasingly difficult, such that in many cases membranes can no
longer even be obtained. It is at present not possible to produce
membranes with high levels of quality and reproducibility.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention was accordingly to provide an
improved method for producing a proton-conducting,
polyazole-containing membrane which does not exhibit the above
disadvantages and permits membranes to be produced in the simplest
possible manner inexpensively and on a large industrial scale. In
particular, the method should require the least possible solvent,
permit production of the membranes with an improved space-time
yield, give rise to membranes of the highest possible quality and
slightest possible batch-to-batch variation, allow the processing
of polyazoles with comparatively high molecular weights and as far
as possible prevent the formation of bubbles in the membrane.
These objects are achieved by a method having all the features of
claim 1.
The present invention accordingly provides a method for producing a
proton-conducting, polyazole-containing membrane, in which A) a
composition containing polyphosphoric acid and at least one
polyazole and exhibiting a solution viscosity in the range from 10
Pas to 1000 Pas, measured to DIN 53018 at the temperature at the
orifice during production of the membrane, is pressed through an
orifice at a temperature in the range from 25.degree. C. to
300.degree. C., and B) the composition is then solidified.
DETAILED DESCRIPTION OF THE INVENTION
The phosphoric acid used comprises conventional commercial
polyphosphoric acid, as is for example obtainable from Riedel-de
Haen. Polyphosphoric acid H.sub.n+2P.sub.nO.sub.3n+1 (n>1)
conventionally has a content, calculated (acidimetrically) as
P.sub.2O.sub.5, of at least 83%.
The polyazole preferably contains azole repeat units of the general
formula (I) and/or (II) and/or (III) and/or (IV) and/or (V) and/or
(VI) and/or (VII) and/or (VIII) and/or (IX) and/or (X) and/or (XI)
and/or (XII) and/or (XIII) and/or (XIV) and/or (XV) and/or (XVI)
and/or (XVII) and/or (XVIII) and/or (XIX) and/or (XX) and/or (XXI)
and/or (XXII)
##STR00001## ##STR00002## ##STR00003## in which Ar are identical or
different and denote a tetravalent aromatic or heteroaromatic
group, which may be mono- or polynuclear, Ar.sup.1 are identical or
different and denote a divalent aromatic or heteroaromatic group,
which may be mono- or polynuclear, Ar.sup.2 are identical or
different and denote a di- or trivalent aromatic or heteroaromatic
group, which may be mono- or polynuclear, Ar.sup.3 are identical or
different and denote a trivalent aromatic or heteroaromatic group,
which may be mono- or polynuclear, Ar.sup.4 are identical or
different and denote a trivalent aromatic or heteroaromatic group,
which may be mono- or polynuclear, Ar.sup.5 are identical or
different and denote a tetravalent aromatic or heteroaromatic
group, which may be mono- or polynuclear, Ar.sup.6 are identical or
different and denote a divalent aromatic or heteroaromatic group,
which may be mono- or polynuclear, Ar.sup.7 are identical or
different and denote a divalent aromatic or heteroaromatic group,
which may be mono- or polynuclear, Ar.sup.8 are identical or
different and denote a trivalent aromatic or heteroaromatic group,
which may be mono- or polynuclear, Ar.sup.9 are identical or
different and denote a di- or tri- or tetravalent aromatic or
heteroaromatic group, which may be mono- or polynuclear, Ar.sup.10
are identical or different and denote a di- or trivalent aromatic
or heteroaromatic group, which may be mono- or polynuclear,
Ar.sup.11 are identical or different and denote a divalent aromatic
or heteroaromatic group, which may be mono- or polynuclear, X is
identical or different and denotes oxygen, sulfur or an amino
group, which bears a hydrogen atom, a group comprising 1-20 carbon
atoms, preferably a branched or unbranched alkyl or alkoxy group,
or an aryl group as a further residue R in all the formulae apart
from formula (XX) identically or differently denotes hydrogen, an
alkyl group or an aromatic group and in formula (XX) denotes an
alkylene group or an aromatic group and n, m is an integer greater
than or equal to 10, preferably greater than or equal to 100.
Preferred aromatic or heteroaromatic groups are derived from
benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane,
diphenyldimethylmethane, bisphenone, diphenyl sulfone, quinoline,
pyridine, bipyridine, pyridazine, pyrimidine, pyrazine, triazine,
tetrazine, pyrrole, pyrazole, anthracene, benzopyrrole,
benzotriazole, benzoxathiadiazole, benzoxadiazole, benzopyridine,
benzopyrazine, benzopyrazidine, benzopyrimidine, benzopyrazine,
benzotriazine, indolizine, quinolizine, pyridopyridine,
imidazopyrimidine, pyrazinopyrimidine, carbazole, aciridine,
phenazine, benzoquinoline, phenoxazine, phenothiazine, acridizine,
benzopteridine, phenanthroline and phenanthrene, which may
optionally also be substituted.
The substitution pattern of Ar.sup.1, Ar.sup.4, Ar.sup.6, Ar.sup.7,
Ar.sup.8, Ar.sup.9, Ar.sup.10, Ar.sup.11 is here at random, for
example in the case of phenylene Ar.sup.1, Ar.sup.4, Ar.sup.6,
Ar.sup.7, Ar.sup.8, Ar.sup.9, Ar.sup.10, Ar.sup.11 may be ortho-,
meta- and para-phenylene. Particularly preferred groups are derived
from benzene and biphenylene, which may optionally also be
substituted.
Preferred alkyl groups are short-chain alkyl groups with 1 to 4
carbon atoms, such as for example methyl, ethyl, n- or i-propyl and
t-butyl groups.
Preferred aromatic groups are phenyl or naphthyl groups. The alkyl
groups and the aromatic groups may be substituted.
Preferred substituents are halogen atoms such as for example
fluorine, amino groups, hydroxyl groups or short-chain alkyl
groups, such as for example methyl or ethyl groups.
Preferred polyazoles are those with repeat units of the formula (I)
in which the residues X are identical within one repeat unit.
The polyazoles may in principle also comprise different repeat
units which differ, for example, in their residue X. Preferably,
however, only identical residues X are present in one repeat
unit.
Further preferred polyazole polymers are polyimidazoles,
polybenzothiazoles, polybenzoxazoles, polyoxadiazoles,
polyquinoxalines, polythiadiazoles, poly(pyridines),
poly(pyrimidines) and poly(tetrazapyrenes).
In a further embodiment of the present invention, the polyazole is
a copolymer which contains at least two units of the formula (I) to
(XXII) which differ from one another. The polymers may also assume
the form of block copolymers (diblock, triblock), random
copolymers, periodic copolymers and/or alternating polymers.
In a particularly preferred embodiment of the present invention,
the polyazole is a homopolymer which contains only units of the
formula (I) and/or (II).
The number of azole repeat units in the polymer is preferably an
integer greater than or equal to 10. Particularly preferred
polymers contain at least 100 azole repeat units.
Polymers containing benzimidazole repeat units are preferred for
the purposes of the present invention. Some examples of the highly
convenient polymers containing benzimidazole repeat units are
represented by the following formulae:
##STR00004## ##STR00005## ##STR00006## wherein n and m are integers
greater than or equal to 10, preferably greater than or equal to
100.
For the purposes of a particularly preferred variant of the present
invention, the polyazoles comprise at least one sulfonic and/or
phosphonic acid group. Such polymers are described in document DE
102 46 459 A1, the disclosure of which is hereby incorporated by
reference.
The polyazoles used, but in particular the polybenzimidazoles, are
distinguished by an elevated molecular weight. Measured as
intrinsic viscosity, this amounts to at least 0.2 dl/g, preferably
0.8 to 10 dl/g, in particular 1 to 10 dl/g.
Preferred polybenzimidazoles are commercially available under the
trade name 0 Celazole.
According to one particularly preferred variant of the method
according to the invention, the polyazoles are produced in situ. To
this end, one or more compounds which, on exposure to heat, are
capable of forming polyazoles may be added the polyphosphoric
acid.
Suitable mixtures are in particular those which comprise one or
more aromatic and/or heteroaromatic tetra-amino compounds and one
or more aromatic and/or heteroaromatic carboxylic acids or the
derivatives thereof comprising at least two acid groups per
carboxylic acid monomer. One or more aromatic and/or heteroaromatic
diaminocarboxylic acids may moreover be used for producing
polyazoles.
The aromatic and heteroaromatic tetra-amino compounds include,
inter alia 3,3',4,4'-tetraminobiphenyl, 2,3,5,6-tetraminopyridine,
1,2,4,5-tetraminobenzene, 3,3',4,4'-tetraminodiphenyl sulfone,
3,3',4,4'-tetraminodiphenyl ether, 3,3',4,4'-tetraminobenzophenone,
3,3',4,4'-tetraminodiphenylmethane and
3,3',4,4'-tetraminodiphenyldimethylmethane and the salts thereof,
in particular the mono-, di-, tri- and tetrahydrochloride
derivatives thereof. Of these, 3,3',4,4'-tetraminobiphenyl,
2,3,5,6-tetraminopyridine and 1,2,4,5-tetraminobenzene, are
particularly preferred.
The mixture may moreover comprise aromatic and/or heteroaromatic
carboxylic acids. These comprise dicarboxylic acids and
tricarboxylic acids and tetracarboxylic acids or the esters thereof
or the anhydrides thereof or the acid halides thereof, in
particular the acid halides and/or acid bromides thereof. The
aromatic dicarboxylic acids preferably comprise isophthalic acid,
terephthalic acid, phthalic acid, 5-hydroxyisophthalic acid,
4-hydroxyisophthalic acid, 2-hydroxyterephthalic acid,
5-aminoisophthalic acid, 5-N,N-dimethylaminoisophthalic acid,
5-N,N-diethylaminoisophthalic acid, 2,5-dihydroxyterephthalic acid,
2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid,
2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid.
3,4-dihydroxyphthalic acid, 3-fluorophthalic acid,
5-fluoroisophthalic acid, 2-fluoroterephthalic acid,
tetrafluorophthalic acid, tetrafluoroisophthalic acid,
tetrafluoroterephthalic acid, 1,4-naphthalenedicarboxylic acid,
1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid,
2,7-naphthalenedicarboxylic acid, diphenic acid,
1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl
ether-4,4'-dicarboxylic acid, benzophenone-4,4'-dicarboxylic acid,
diphenyl sulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic
acid, 4-trifluoromethylphthalic acid,
2,2-bis(4-carboxyphenyl)hexafluoropropane,
4,4'-stilbenedicarboxylic acid, 4-carboxycinnamic acid, or the
C1-C20 alkyl esters or C5-C12 aryl esters thereof, or the acid
anhydrides thereof or the acid chlorides thereof.
The aromatic tricarboxylic acids or the C1-C20 alkyl esters or
C5-C12 aryl esters thereof or the acid anhydrides thereof or the
acid chlorides thereof preferably comprise
1,3,5-benzeneltricarboxylic acid (trimesic acid),
1,2,4-benzeneltricarboxylic acid (trimellitic acid),
(2-carboxyphenyl)iminodiacetic acid, 3,5,3'-biphenyltricarboxylic
acid, 3,5,4'-biphenyltricarboxylic acid.
The aromatic tetracarboxylic acids or the C1-C20 alkyl esters or
C5-C12 aryl esters thereof or the acid anhydrides thereof or the
acid chlorides thereof preferably comprise
3,5,3',5'-biphenyltetracarboxylic acid,
1,2,4,5-benzenetetracarboxylic acid, benzophenonetetracarboxylic
acid, 3,3',4,4'-biphenyltetracarboxylic acid,
2,2',3,3'-biphenyltetracarboxylic acid,
1,2,5,6-naphthalenetetracarboxylic acid,
1,4,5,8-naphthalenetetracarboxylic acid.
The heteroaromatic carboxylic acids preferably comprise
heteroaromatic dicarboxylic acids and tricarboxylic acids and
tetracarboxylic acids or the esters thereof or the anhydrides
thereof. Heteroaromatic carboxylic acids are taken to be aromatic
systems which contain at least one nitrogen, oxygen, sulfur or
phosphorus atom in the aromatic moiety. They preferably comprise
pyridine-2,5-dicarboxylic acid, pyridine-3,5-dicarboxylic acid,
pyridine-2,6-dicarboxylic acid, pyridine-2,4-dicarboxylic acid,
4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic
acid, 2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic
acid, 2,4,6-pyridinetricarboxylic acid,
benzimidazole-5,6-dicarboxylic acid together with the C1-C20 alkyl
esters or C5-C12 aryl esters thereof, or the acid anhydrides
thereof or the acid chlorides thereof.
The content of tricarboxylic acid or tetracarboxylic acids
(relative to the introduced dicarboxylic acid) amounts to between 0
and 30 mol %, preferably 0.1 and 20 mol %, in particular 0.5 and 10
mol %.
Aromatic and heteroaromatic diaminocarboxylic acids may furthermore
also be used. These include inter alia diaminobenzoic acid,
4-phenoxycarbonyl-3,'4'-diaminodiphenyl ether and the mono- and
dihydrochloride derivatives thereof.
Preferably, mixtures of at least 2 different aromatic carboxylic
acids are used. Mixtures which are particularly preferably used are
those which, in addition to aromatic carboxylic acids, also contain
heteroaromatic carboxylic acids. The mixing ratio of aromatic
carboxylic acids to heteroaromatic carboxylic acids amounts to
between 1:99 and 99:1, preferably between 1:50 to 50:1.
These mixtures in particular comprise mixtures of N-heteroaromatic
dicarboxylic acids and aromatic dicarboxylic acids. Non-limiting
examples of dicarboxylic acids are isophthalic acid, terephthalic
acid, phthalic acid, 2,5-dihydroxyterephthalic acid,
2,6-dihydroxyisophthalic acid, 4,6-dihydroxyisophthalic acid,
2,3-dihydroxyphthalic acid, 2,4-dihydroxyphthalic acid,
3,4-dihydroxyphthalic acid, 1,4-naphthalenedicarboxylic acid,
1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid,
2,7-naphthalenedicarboxylic acid, diphenic acid,
1,8-dihydroxynaphthalene-3,6-dicarboxylic acid, diphenyl
ether-4,4'-dicarboxylic acid, benzophenone-4,4'-dicarboxylic acid,
diphenyl sulfone-4,4'-dicarboxylic acid, biphenyl-4,4'-dicarboxylic
acid, 4-trifluoromethylphthalic acid, pyridine-2,5-dicarboxylic
acid, pyridine-3,5-dicarboxylic acid, pyridine-2,6-dicarboxylic
acid, pyridine-2,4-dicarboxylic acid,
4-phenyl-2,5-pyridinedicarboxylic acid, 3,5-pyrazoledicarboxylic
acid, 2,6-pyrimidinedicarboxylic acid, 2,5-pyrazinedicarboxylic
acid.
If a molecular weight which is as high as possible is to be
achieved, the molar ratio of carboxylic acid groups to amino groups
during the reaction of tetra-amino compounds with one or more
aromatic carboxylic acids or the esters thereof, which contain at
least two acid groups per carboxylic acid monomer, is preferably in
the vicinity of 1:2.
Preferably at least 0.5 wt. %, in particular 1 to 30 wt. % and
particularly preferably 2 to 15 wt. % of monomers are used to
produce polyazoles, in each case relative to the resultant weight
of the composition to be used.
If the polyazoles are produced from the monomers directly in the
polyphosphoric acid, the polyazoles are distinguished by an
elevated molecular weight. This is particularly the case for
polybenzimidazoles. Measured as intrinsic viscosity, this is in the
range from 0.3 to 10 dl/g, preferably in the range from 1 to 5
dl/g.
Where tricarboxylic acids or tetracarboxylic acid are also used,
they give rise to branching/crosslinking of the resultant polymers.
This contributes an improvement in mechanical properties.
According to a further aspect of the present invention, compounds
are used which, on exposure to heat, are suitable for forming
polyazoles, wherein these compounds are obtainable by reacting one
or more aromatic and/or heteroaromatic tetra-amino compounds with
one or more aromatic and/or heteroaromatic carboxylic acids or the
derivatives thereof, which contain at least two acid groups per
carboxylic acid monomer, or one or more aromatic and/or
heteroaromatic diaminocarboxylic acids in a melt at temperatures of
up to 400.degree. C., in particular of up to 350.degree. C.,
preferably of up to 280.degree. C. The compounds to be used for
producing these prepolymers have been explained above.
In principle, no restrictions apply to the particular proportions
of the polyphosphoric acid and the polyazole. Compositions which
are particularly suitable for the purposes of the present invention
contain, in each case relative to the total weight (initial weight)
thereof, 70.0 wt. % to 99.999 wt. %, preferably 90.0 wt. % to 99.9
wt. %, particularly preferably 95.0 wt. % to 99.5 wt. %, of
polyphosphoric acid and 30.0 wt. % to 0.001 wt. %, preferably 10.0
wt. % to 0.1 wt. %, particularly preferably 5.0 wt. % to 0.5 wt. %,
of polyazole.
The composition to be used in the present method preferably assumes
the form of a dispersion, suspension or solution and may optionally
comprise a low solid content and/or gel content. Particularly
preferably, however, the proportion of constituents which can be
filtered out is less than 30.0 wt. %, preferably less than 10.0 wt.
%, in particular less than 5.0 wt. %, in each case relative to the
total weight of the composition. Determination of the quantities
which can be filtered out here favourably proceeds at the
temperature at which the method according to the invention is
carried out (temperature at the orifice). Furthermore, screens with
screen openings (mesh) preferably of less than 1.0 mm, preferably
of less than 500 .mu.m, particularly preferably of less than 100
.mu.m, are used.
The composition to be used in the method exhibits a solution
viscosity in the range from 10 Pas to 1000 Pas, preferably in the
range from 20 Pas to 800 Pas, particularly preferably in the range
from 30 Pas to 600 Pas, in particular in the range from 50 Pas to
500 Pas. Solution viscosity is measured according to DIN 53018 at a
shear rate of 30 Hz between two 20 mm plates. Viscosity is measured
at the temperature which corresponds to the temperature at the
orifice during production of the membrane.
The polyazole in the composition furthermore favourably exhibits an
intrinsic viscosity (IV) in at least 96% sulfuric acid of 0.3 to
10, particularly preferably of 1 to 5. The intrinsic viscosity may
here be determined in per se known manner by measuring
concentration series and extrapolating to infinite dilution. The
measurements are preferably made at a temperature of between
0.degree. C. and 100.degree. C., particularly preferably of between
20.degree. C. and 80.degree. C., in particular at 25.degree. C. It
has furthermore proved particularly effective to use Ostwald
viscosimeters and/or Ubbelohde viscosimeters.
Further information regarding viscosity parameters and the
associated determination methods may be found in the usual
specialist literature, for example Ullmann 1, 67-85; (4th ed.) 5,
755-778, the disclosure of which is hereby incorporated by
reference.
For the purposes of a highly preferred variant of the present
invention, the composition to be used furthermore contains at least
one polymer which is not a polyazole (polymer B). These polymers
may inter alia assume dissolved, dispersed or suspended form.
The weight ratio of polyazole to polymer (B) is here preferably in
the range from 0.1 to 50, preferentially in the range from 0.2 to
20, particularly preferably in the range from 1 to 10. If the
polyazole is formed in situ, the weight ratio may be obtained by
calculation from the weight of the monomers for forming the
polyazole, wherein the compounds, for example water, liberated
during condensation must be taken into account.
Preferred polymers include inter alia polyolefins, such as
poly(chloroprene), polyacetylene, polyphenylene, poly(p-xylylene),
polyarylmethylene, polyarmethylene, polystyrene, polymethylstyrene,
polyvinyl alcohol, polyvinyl acetate, polyvinyl ether,
polyvinylamine, poly(N-vinylacetamide), polyvinylimidazole,
polyvinylcarbazole, polyvinylpyrrolidone, polyvinylpyridine,
polyvinyl chloride, polyvinylidene chloride,
polytetrafluoroethylene, polyhexafluoropropylene, copolymers of
PTFE with hexafluoropropylene, with perfluoropropyl vinyl ether,
with trifluoronitrosomethane, with sulfonyl fluoride vinyl ether,
with carbalkoxyperfluoroalkoxy vinyl ether,
polychlorotrifluoroethylene, polyvinyl fluoride, polyvinylidene
fluoride, polyacrolein, polyacrylamide, polyacrylonitrile,
polycyanoacrylates, polymethacrylimide, cycloolefinic copolymers,
in particular prepared from norbornene; polymers with C--O bonds in
the main chain, for example polyacetal, polyoxymethylene,
polyether, polypropylene oxide, polyepichlorohydrin,
polytetrahydrofuran, polyphenylene oxide, polyether ketone,
polyester, in particular polyhydroxyacetic acid, polyethylene
terephthalate, polybutylene terephthalate, polyhydroxy benzoate,
polyhydroxypropionic acid, polypivalolactone, polycaprolactone,
polymalonic acid, polycarbonate;
polymers with C--S bonds in the main chain, for example polysulfide
ethers, polyphenylene sulfide, polyether sulfone;
polymers with C--N bonds in the main chain, for example polyimines,
polyisocyanides, polyether imine, polyaniline, polyamides,
polyhydrazides, polyurethanes, polyimides, polyazoles,
polyazines;
liquid crystal polymers, in particular Vectra and
inorganic polymers, for example polysilanes, polycarbosilanes,
polysiloxanes, polysilicic acid, polysilicates, silicones,
polyphosphazenes and polythiazyl.
Moreover, polymers with covalently attached acid groups are also
among preferred polymers (B). These acid groups in particular
comprise sulfonic acid groups. The polymers modified with sulfonic
acid groups preferably have a content of sulfonic acid groups in
the range from 0.5 to 3 meq/g. This value is determined by means of
the "ion exchange capacity" (IEC).
The IEC is measured by converting the sulfonic acid groups into the
free acid. To this end, the polymer is treated in known manner with
acid, any excess acid being removed by washing. The sulfonated
polymer is accordingly initially treated for 2 hours in boiling
water. Excess water is then blotted off and the sample dried for 15
hours at 160.degree. C. in a vacuum drying cabinet at p<1 mbar.
The dry weight of the membrane is then determined. The polymer
dried in this manner is then dissolved in DMSO at 80.degree. C. for
1 h. The solution is then titrated with 0.1 M NaOH. The ion
exchange capacity (IEC) is then calculated from the quantity of
acid consumed to reach the equivalence point and the dry
weight.
Such polymers are known to those skilled in the art. Polymers
containing sulfonic acid groups may accordingly be produced, for
example, by sulfonating polymers. Method for sulfonating polymers
are described in F. Kucera et. al. Polymer Engineering and Science
1988, Vol. 38, No. 5, 783-792. Sulfonation conditions may here be
selected such that a low degree of sulfonation is obtained
(DE-A-19959289).
A further class of non-fluorinated polymers has accordingly been
developed by sulfonating high temperature resistant thermoplastics.
Sulfonated polyether ketones (DE-A-4219077, WO96/01177), sulfonated
polysulfones (J. Membr. Sci. 83 (1993) p. 211) or sulfonated
polyphenylene sulfide (DE-A-19527435) are accordingly known.
U.S. Pat. No. 6,110,616 describes copolymers of butadiene and
styrene and the subsequent sulfonation thereof for fuel cell
use.
Such polymers may moreover also be obtained by polyreactions of
monomers comprising acid groups. Perfluorinated polymers as
described in U.S. Pat. No. 5,422,411 may accordingly be produced by
copolymerisation from trifluorostyrene and sulfonyl-modified
trifluorostyrene.
One such perfluorosulfonic acid polymer is inter alia
Nafion.degree. (U.S. Pat. No. 3,692,569). This polymer may be
dissolved as described in U.S. Pat. No. 4,453,991 and then used as
an ionomer.
Preferred polymers with acid groups include inter alia sulfonated
polyether ketones, sulfonated polysulfones, sulfonated
polyphenylene sulfides, perfluorinated polymers containing sulfonic
acid groups, as described in U.S. Pat. No. 3,692,569, U.S. Pat. No.
5,422,411 and U.S. Pat. No. 6,110,616.
Polymers (B) which are preferred for use in fuel cells with a
continuous service temperature of above 100.degree. C. are those
which have a glass transition temperature or Vicat softening
temperature VSTIA/50 of at least 100.degree. C., preferably of at
least 150.degree. C. and very particularly preferably of at least
180.degree. C.
Polysulfones with a Vicat softening temperature VST/A/50 of
180.degree. C. to 230.degree. C. are here preferred.
Preferred polymers (B) are furthermore those which exhibit slight
solubility and/or degradability in phosphoric acid. According to
one particular embodiment of the present invention, treatment with
85% phosphoric acid brings about only insignificant weight loss.
The weight ratio of the plate after phosphoric acid treatment to
the weight of the plate before treatment is preferably greater than
or equal to 0.8, in particular greater than or equal to 0.9 and
particularly preferably greater than or equal to 0.95. This value
is measured on a plate of polymer (B) which is 2 mm thick, 5 cm
long and 2 cm wide. This plate is placed in phosphoric acid, the
weight ratio of phosphoric acid to plate amounting to 10. The
phosphoric acid is then heated to 100.degree. C. with stirring for
24 hours. Any excess phosphoric acid is then removed from the plate
by washing with water and the plate is dried. The plate is then
reweighed.
Preferred polymers include polysulfones, in particular polysulfone
with aromatic moieties in the main chain. According to one
particular aspect of the present invention, preferred polysulfones
and polyether sulfones exhibit a melt volume rate MVR 300/21.6,
measured to 180 1133, of less than or equal to 40 cm.sup.3/10 min,
in particular of less than or equal to 30 cm.sup.3/10 min and
particularly preferably of less than or equal to 20 cm.sup.3/10
min.
The mixture is polymerised by being heated to a temperature of up
to 400.degree. C., in particular of 350.degree. C., preferably of
up to 280.degree. C., in particular of 100.degree. C. to
250.degree. C. and particularly preferably in the range from
200.degree. C. to 250.degree. C. An inert gas, for example
nitrogen, or a noble gas, such as neon or argon, is used here.
Applicational properties may be further improved by also adding
fillers, in particular proton-conducting fillers, and additional
acids to the composition.
Non-limiting examples of proton-conducting fillers are sulfates
such as: CsHSO.sub.4, Fe(SO.sub.4).sub.2,
(NH.sub.4).sub.3H(SO.sub.4).sub.2, LiHSO.sub.4, NaHSO.sub.4,
KHSO.sub.4, RbSO.sub.4, LiN.sub.2H.sub.5SO.sub.4,
NH.sub.4HSO.sub.4, phosphates such as Zr.sub.3(PO.sub.4).sub.4,
Zr(HPO.sub.4).sub.2, HZr.sub.2(PO.sub.4).sub.3,
UO.sub.2PO.sub.4.3H.sub.2O, H.sub.8UO.sub.2PO.sub.4,
Ce(HPO.sub.4).sub.2, Ti(HPO.sub.4).sub.2, KH.sub.2PO.sub.4,
NaH.sub.2PO.sub.4, LiH.sub.2PO.sub.4, NH.sub.4H.sub.2PO.sub.4,
CsH.sub.2PO.sub.4, CaHPO.sub.4, MgHPO.sub.4, HSbP.sub.2O.sub.8,
HSb.sub.3P.sub.2O.sub.14, H.sub.5Sb.sub.5P.sub.2O.sub.20, polyacid
such as H.sub.3PW.sub.12O.sub.40.nH.sub.2O (n=21-29),
H.sub.3SiW.sub.12O.sub.40.nH.sub.2O (n=21-29), H.sub.xWO.sub.3,
HSbWO.sub.6, H.sub.3PMo.sub.12O.sub.40, H.sub.2Sb.sub.4O.sub.11,
HTaWO.sub.6, HNbO.sub.3, HTiNbO.sub.5, HTiTaO.sub.5, HSbTeO.sub.6,
H.sub.5Ti.sub.4O.sub.9, HSbO.sub.3, H.sub.2MoO.sub.4 selenites and
arsenides such as (NH.sub.4).sub.3H(SeO.sub.4).sub.2,
UO.sub.2AsO.sub.4, (NH.sub.4).sub.3H(SeO.sub.4).sub.2,
KH.sub.2AsO.sub.4, Cs.sub.3H(SeO.sub.4).sub.2,
Rb.sub.3H(SeO.sub.4).sub.2, oxides such as Al.sub.2O.sub.3,
Sb.sub.2O.sub.5, ThO.sub.2, SnO.sub.2, ZrO.sub.2, MoO.sub.3
silicates such as zeolites, zeolites(NH.sub.4.sup.+),
phyllosilicates, tectosilicates, H-natrolites, H-mordenites,
NH.sub.4-analcines, NH.sub.4-sodalites, NH.sub.4-gallates,
H-montmorillonites acids such as HClO.sub.4, SbF.sub.5 fillers such
as carbides, in particular SiC, Si.sub.3N.sub.4, fibres, in
particular glass fibres, glass powders and/or polymer fibres,
preferably based on polyazoles.
These additives may be present in the composition in conventional
quantities, but the positive properties of the membrane, such as
elevated conductivity, long life span and elevated mechanical
stability should not be impaired too much by adding excessively
large quantities of additives. In general, the resultant membrane
comprises at most 80 wt. %, preferably at most 50 wt. % and
particularly preferably at most 20 wt. % of additives.
The composition may furthermore also contain perfluorinated
sulfonic acid additives (preferably 0.1-20 wt. %, preferentially
0.2-15 wt. %, highly preferably 0.2-10 wt. %). These additives
enhance performance, in the vicinity of the cathode increasing
oxygen solubility and oxygen diffusion and reducing adsorption of
phosphoric acid and phosphate onto platinum. (Electrolyte additives
for phosphoric acid fuel cells. Gang, Xiao; Hjuler, H. A.; Olsen,
C.; Berg, R. W.; Bjerrum, N.J. Chem. Dep. A, Tech, Univ. Denmark,
Lyngby, Den. J. Electrochem. Soc. (1993), 140(4), 896-902 and
Perfluorosulfonimide as an additive in phosphoric acid fuel cell.
Razaq, M.; Razaq, A.; Yeager, E.; DesMarteau, Darryl D.; Singh, S.
Case Cent. Electrochem. Sci., Case West. Reserve Univ., Cleveland,
Ohio, USA. J. Electrochem. Soc. (1989), 136(2), 385-90.)
Non-limiting examples of persulfonated additives are:
trifluoromethanesulfonic acid, potassium trifluoromethanesulfonate,
sodium trifluoromethanesulfonate, lithium
trifluoromethanesulfonate, ammonium trifluoromethanesulfonate,
potassium perfluorohexanesulfonate, sodium
perfluorohexanesulfonate, lithium perfluorohexanesulfonate,
ammonium perfluorohexanesulfonate, perfluorohexanesulfonic acid,
potassium nonafluorobutanesulfonate, sodium
nonafluorobutanesulfonate, lithium nonafluorobutanesulfonate,
ammonium nonafluorobutanesulfonate, caesium
nonafluorobutanesulfonate, triethylammonium
perfluorohexanesulfonate, perfluorosulfoimides and Nafion.
In the method according to the invention, the composition
containing polyphosphoric acid and at least one polyazole is
pressed through a orifice, preferably a die. A die is a constantly
forward tapering part through which the solution is passed.
The orifice is preferably of slot-shaped configuration and has an
elongate, narrow, preferably rectangular shape. The width of the
slot is preferably in the range from 10 cm to 2.0 m, particularly
preferably in the range from 20 cm to 50 cm. The slot gap is
favourably in the range from 250 .mu.m to 5 mm, in particular in
the range from 800 .mu.m to 1600 .mu.m. The ratio of slot width to
slot gap is preferably in the range from 10:1 to 10000:1,
particularly preferably in the range from 100:1 to 1000:1.
The method is carried out at a temperature, measured at the
orifice, in the range from 25.degree. C. to 300.degree. C.,
preferably in the range from 120.degree. C. to 180.degree. C.,
particularly preferably in the range from 165.degree. C. to
170.degree. C.
The other method parameters may in principle be freely selected and
adapted to the particular individual case. It has nevertheless
proved particularly favourable to adjust the pressure at which the
solution is pressed through the orifice to a value of at least 0.1
bar, preferably of at least 1.5 bar, in particular to a value in
the range from 2 bar to 2.5 bar.
For the purposes of a very particularly preferred embodiment, the
orifice is part of an extruder, through which the solution is
extruded. It has proved very particularly effective to use single
screw extruders or twin screw extruders in this connection.
It is furthermore very particularly favourable according to the
invention to mix the polyazole and the polyphosphoric acid in the
extruder. If the polyazole is to be produced in situ in the
extruder, the extruder may comprise zones with different
temperatures (synthesis zone, orifice). All that is essential is
that the criteria according to the invention, i.e. the solution
viscosity and the extrusion temperature at the orifice, are
observed.
The composition pressed through the die is preferably applied onto
a support, in particular onto an electrode. The extruded
composition is then solidified.
For the purposes of a particularly preferred variant of the present
invention, the composition is applied onto the support in such a
manner that a plurality of non-interconnected fields of the
composition is formed on the support. This may, for example, be
achieved by pressing the composition through a divided gap or by
interrupting application onto the support for a brief period and
continuing at another point. One advantage of this procedure is
inter alis the possibility of directly tailoring the shape of the
membrane to requirements and avoiding subsequent cutting to
size.
Solidification according to the invention of the extruded membrane
proceeds by cooling, for example by air quenching, and/or a
post-treatment (hydrolysis and/or crosslinking), preferably with a
liquid or a liquid mixture, preferably in a dip bath containing the
above-stated liquids.
The above-stated liquids adjust the temperature of the extruded
membrane to a preselected temperature range, such that cooling
and/or post-treatment (hydrolysis and/or crosslinking) may proceed
over a preselected temperature profile.
The extruded membrane obtained by the method is preferably
subjected to post-treatment with moisture, such that the
polyphosphoric acid which is present is at least partially
hydrolysed to form low molecular weight polyphosphoric acid and/or
phosphoric acid. The membrane is preferably treated at temperatures
in the range between -100.degree. C. and 150.degree. C., preferably
at temperatures between 10.degree. C. and 120.degree. C., in
particular between room temperature (20.degree. C.) and 110.degree.
C., particularly preferably between 30.degree. C. and 100.degree.
C. Treatment furthermore preferably proceeds under normal pressure,
but may also proceed with exposure to pressure. For the purposes of
a particularly preferred variant of the present invention, the
membrane is passed through a bath which comprises water or aqueous
liquids.
Hydrolysis of the polyphosphoric acid brings about solidification
of the composition and a decrease in the film thickness and
formation of a membrane. The solidified membrane generally has a
thickness of between 15 and 3000 .mu.m, preferably 20 and 2000
.mu.m, in particular between 20 and 1500 .mu.m, the membrane being
self-supporting.
The upper temperature limit for moisture treatment is generally
150.degree. C. In the case of extremely brief exposure to moisture,
for example to superheated steam, said steam may also be hotter
than 150.degree. C. The upper temperature limit is substantially
determined by the duration of the treatment.
Hydrolysis may also proceed in conditioning cabinets, in which
hydrolysis may be purposefully controlled with defined exposure to
moisture. The moisture content may here be adjusted by the
temperature or saturation of the contacting environment, for
example gases such as air, nitrogen, carbon dioxide or other
suitable gases, or steam. Treatment time is dependent on the
above-selected parameters.
Treatment time is furthermore dependent on the thickness of the
membrane.
The treatment time generally amounts to between a few seconds to
minutes, for example in the case of exposure to superheated steam,
or up to whole days, for example in air at room temperature and low
relative atmospheric humidity. The treatment time preferably
amounts to between 10 seconds and 300 hours, in particular 1 minute
to 200 hours.
If hydrolysis is performed at room temperature (20.degree. C.) with
ambient air of a relative atmospheric humidity of 40-80%, the
treatment time preferably amounts to between 1 and 200 hours.
The resultant membrane may be made self-supporting, i.e. it can be
detached from the support without suffering damage and then
optionally be directly further processed.
The concentration of phosphoric acid and thus the conductivity of
the polymer membranes according to the invention may be adjusted by
the degree of hydrolysis, i.e. duration, temperature and ambient
humidity. According to the invention, the concentration of
phosphoric acid is stated as mol of acid per mol of polymer repeat
unit. For the purposes of the present invention, the concentration
(mol of phosphoric acid relative to one repeat unit of the formula
(III), i.e. polybenzimidazole) is between 10 and 90, in particular
between 12 and 85, particularly preferably between 15 and 80, mol
of phosphoric acid. Such high doping rates (concentrations) can be
achieved only with great difficulty, if at all, by doping
polyazoles with commercially obtainable orthophosphoric acid.
In a further embodiment of the invention, the liquids or liquid
mixtures stated for post-treatment comprise those which enable
hydrolysis of the polyphosphoric acid (hydrolysis liquid).
The extruded membrane is treated at the above-stated temperatures.
In addition to water, the hydrolysis liquid also comprises at least
one oxo acid of phosphorus and/or sulfur. In this case too,
treatment preferably proceeds under normal pressure, but may also
proceed with exposure to pressure.
The hydrolysis liquid may be a solution, the liquid possibly also
containing suspended and/or dispersed constituents. The viscosity
of the hydrolysis liquid may vary over wide ranges, it being
possible to adjust the viscosity by adding solvents or increasing
temperature. Dynamic viscosity is preferably in the range from 0.1
to 10000 mPas, in particular 0.2 to 2000 mPas, it being possible to
measure these values, for example, according to DIN 53015.
Post-treatment may proceed by any known method. For example, the
membrane may be immersed in a liquid bath or be sprayed with the
hydrolysis liquid. The hydrolysis liquid may also be poured over
the membrane.
Oxo acids of phosphorus and/or sulfur include in particular
phosphinic acid, phosphonic acid, phosphoric acid, hypodiphosphonic
acid, hypodiphosphoric acid, oligophosphoric acids, sulfurous acid,
disulfurous acid and/or sulfuric acid. These acids may be used
individually or as a mixture.
The oxo acids of phosphorus and/or sulfur moreover comprise
free-radically polymerisable monomers comprising phosphonic acid
and/or sulfonic acid groups. In this embodiment, the liquid,
provided that it contains water, may effect hydrolysis and, on the
other hand, effect solidification by subsequent polymerisation of
the monomers. Providing that the post-treatment liquid also
comprises compounds capable of crosslinking, solidification may
also proceed by crosslinking.
Phosphonic acid groups comprising monomers are known to those
skilled in the art. These are compounds which comprise at least one
carbon-carbon double bond and at least one phosphonic acid group.
The two carbon atoms which form the carbon-carbon double bond
preferably comprise at least two, preferably 3, bonds to groups
which result in low steric inhibition of the double bond. These
groups include inter alia hydrogen atoms and halogen atoms, in
particular fluorine atoms. For the purposes of the present
invention, the polymer comprising phosphonic acid groups arises
from the polymerisation product which is obtained by polymerisation
of the monomer comprising phosphonic acid groups alone or with
further monomers and/or crosslinking agents.
The monomer comprising phosphonic acid groups may comprise one,
two, three or more carbon-carbon double bonds. The monomer
comprising phosphonic acid groups may furthermore contain one, two,
three or more phosphonic acid groups.
In general, the monomer comprising phosphonic acid groups contains
2 to 20, preferably 2 to 10 carbon atoms.
The monomer comprising phosphonic acid groups preferably comprises
compounds of the formula
##STR00007## in which R means a bond, a divalent C1-C15 alkylene
group, divalent C1-C15 alkyleneoxy group, for example ethyleneoxy
group or divalent C5-C20 aryl or heteroaryl group, wherein the
above residues may in turn be substituted with halogen, --OH, COOZ,
--CN, NZ.sub.2, Z mutually independently means hydrogen, C1-C15
alkyl group, C1-C15 alkoxy group, ethyleneoxy group or C5-C20 aryl
or heteroaryl group, wherein the above residues may in turn be
substituted with halogen, --OH, --CN, and x means an integer 1, 2,
3, 4, 5, 6, 7, 8, 9 or 10 y means an integer 1, 2, 3, 4, 5, 6, 7,
8, 9 or 10 and/or of the formula
##STR00008## in which R means a bond, a divalent C1-C15 alkylene
group, divalent C1-C15 alkyleneoxy group, for example ethyleneoxy
group or divalent C5-C20 aryl or heteroaryl group, wherein the
above residues may in turn be substituted with halogen, --OH, COOZ,
--CN, NZ.sub.2, Z mutually independently means hydrogen, C1-C15
alkyl group, C1-C15 alkoxy group, ethyleneoxy group or C5-C20 aryl
or heteroaryl group, wherein the above residues may in turn be
substituted with halogen, --OH, --CN, and x means an integer 1, 2,
3, 4, 5, 6, 7, 8, 9 or 10 and/or of the formula
##STR00009## in which A represents a group of the formulae
COOR.sup.2, CN, CONR.sup.2.sub.2, OR.sup.2 and/or R.sup.2, in which
R.sup.2 means hydrogen, a C1-C15 alkyl group, C1-C15 alkoxy group,
ethyleneoxy group or C5-C20 aryl or heteroaryl group, wherein the
above residues may in turn be substituted with halogen, --OH, COOZ,
--CN, NZ.sub.2 R means a bond, a divalent C1-C15 alkylene group,
divalent C1-C15 alkyleneoxy group, for example ethyleneoxy group or
divalent C5-C20 aryl or heteroaryl group, wherein the above
residues may in turn be substituted with halogen, --OH, COOZ, --CN,
NZ.sub.2, Z mutually independently means hydrogen, C1-C15 alkyl
group, C1-C15 alkoxy group, ethyleneoxy group or C5-C20 aryl or
heteroaryl group, wherein the above residues may in turn be
substituted with halogen, --OH, --CN, and x means an integer 1, 2,
3, 4, 5, 6, 7, 8, 9 or 10.
Preferred monomers comprising phosphonic acid groups include inter
alia alkenes which comprise phosphonic acid groups, such as
ethenephosphonic acid, propenephosphonic acid, butenephosphonic
acid; acrylic acid and/or methacrylic acid compounds which comprise
phosphonic acid groups, such as for example
2-phosphonomethylacrylic acid, 2-phosphonomethylmethacrylic acid,
2-phosphonomethylacrylamide and
2-phosphonomethylmethacrylamide.
Conventional commercial vinylphosphonic acid (ethenephosphonic
acid), as is obtainable for example from Aldrich or Clariant GmbH,
is particularly preferably used. A preferred vinylphosphonic acid
exhibits a purity of greater than 70%, in particular 90% and
particularly preferably greater than 97% purity.
The monomers comprising phosphonic acid groups may moreover also be
used in the form of derivatives, which may then be converted into
the acid, the conversion into the acid also possibly proceeding in
the polymerised state. These derivatives include in particular the
salts, esters, amides and halides of the monomers comprising
phosphonic acid groups.
The monomers comprising phosphonic acid groups may moreover also be
introduced onto and into the membrane after hydrolysis. This may
proceed by means of per se known measures (for example spraying,
dipping etc.) which are known from the prior art.
According to one particular aspect of the present invention, the
ratio of the weight of the sum of phosphoric acid, polyphosphoric
acid and the hydrolysis products of polyphosphoric acid to the
weight of the free-radically polymerisable monomers, for example of
the monomers comprising phosphonic acid groups, is preferably
greater than or equal 1:2, in particular greater than or equal 1:1.
and particularly preferably greater than or equal 2:1.
The ratio of the weight of the sum of phosphoric acid,
polyphosphoric acid and the hydrolysis products of polyphosphoric
acid to the weight of the free-radically polymerisable monomers is
in the range from 1000:1 to 3:1, in particular 100:1 to 5:1 and
particularly preferably 50:1 to 10:1.
This ratio may readily be determined by conventional methods, it
often being possible to wash the phosphoric acid, polyphosphoric
acid and the hydrolysis products thereof out of the membrane. The
weight of the polyphosphoric acid and the hydrolysis products
thereof after complete hydrolysis may here be related to the
phosphoric acid. This generally likewise applies to the
free-radically polymerisable monomers.
Monomers comprising sulfonic acid groups are known to those skilled
in the art. These are compounds which comprise at least one
carbon-carbon double bond and at least one sulfonic acid group. The
two carbon atoms which form the carbon-carbon double bond
preferably comprise at least two, preferably 3, bonds to groups
which result in low steric inhibition of the double bond. These
groups include inter alia hydrogen atoms and halogen atoms, in
particular fluorine atoms. For the purposes of the present
invention, the polymer comprising sulfonic acid groups arises from
the polymerisation product which is obtained by polymerisation of
the monomer containing sulfonic acid groups alone or with further
monomers and/or crosslinking agents.
The monomer comprising sulfonic acid groups may comprise one, two,
three or more carbon-carbon double bonds. The monomer comprising
sulfonic acid groups may furthermore contain one, two, three or
more sulfonic acid groups.
In general, the monomer comprising sulfonic acid groups contains 2
to 20, preferably 2 to 10 carbon atoms.
The monomer comprising sulfonic acid groups preferably comprises
compounds of the formula
##STR00010## in which R means a bond, a divalent C1-C15 alkylene
group, divalent C1-C15 alkyleneoxy group, for example ethyleneoxy
group or divalent C5-C20 aryl or heteroaryl group, wherein the
above residues may in turn be substituted with halogen, --OH, COOZ,
--CN, NZ.sub.2, Z mutually independently means hydrogen, C1-C15
alkyl group, C1-C15 alkoxy group, ethyleneoxy group or C5-C20 aryl
or heteroaryl group, wherein the above residues may in turn be
substituted with halogen, --OH, --CN, and x means an integer 1, 2,
3, 4, 5, 6, 7, 8, 9 or 10 y means an integer 1, 2, 3, 4, 5, 6, 7,
8, 9 or 10 and/or of the formula
##STR00011## in which R means a bond, a divalent C1-C15 alkylene
group, divalent C1-C15 alkyleneoxy group, for example ethyleneoxy
group or divalent C5-C20 aryl or heteroaryl group, wherein the
above residues may in turn be substituted with halogen, --OH, COOZ,
--CN, NZ.sub.2, Z mutually independently means hydrogen, C1-C15
alkyl group, C1-C15 alkoxy group, ethyleneoxy group or C5-C20 aryl
or heteroaryl group, wherein the above residues may in turn be
substituted with halogen, --OH, --CN, and x means an integer 1, 2,
3, 4, 5, 6, 7, 8, 9 or 10 and/or of the formula
##STR00012## in which A represents a group of the formulae
COOR.sup.2, CN, CONR.sup.2.sub.2, OR.sup.2 and/or R.sup.2, in which
R.sup.2 means hydrogen, a C1-C15 alkyl group, C1-C15 alkoxy group,
ethyleneoxy group or C5-C20 aryl or heteroaryl group, wherein the
above residues may in turn be substituted with halogen, --OH, COOZ,
--CN, NZ.sub.2 R means a bond, a divalent C1-C15 alkylene group,
divalent C1-C15 alkyleneoxy group, for example ethyleneoxy group or
divalent C5-C20 aryl or heteroaryl group, wherein the above
residues may in turn be substituted with halogen, --OH, COOZ, --CN,
NZ.sub.2, Z mutually independently means hydrogen, C1-C15 alkyl
group, C1-C15 alkoxy group, ethyleneoxy group or C5-C20 aryl or
heteroaryl group, wherein the above residues may in turn be
substituted with halogen, --OH, --CN, and x means an integer 1, 2,
3, 4, 5, 6, 7, 8, 9 or 10.
Preferred monomers comprising sulfonic acid groups inter alia
include alkenes which comprise sulfonic acid groups, such as
ethenesulfonic acid, propenesulfonic acid, butenesulfonic acid;
acrylic acid and/or methacrylic acid compounds which comprise
sulfonic acid groups, such as for example 2-sulfonomethylacrylic
acid, 2-sulfonomethylmethacrylic acid, 2-sulfonomethylacrylamide
and 2-sulfonomethylmethacrylamide.
Conventional commercial vinylsulfonic acid (ethenesulfonic acid),
as is obtainable for example from Aldrich or Clariant GmbH, is
particularly preferably used. A preferred vinylsulfonic acid
exhibits a purity of greater than 70%, in particular 90% and
particularly preferably greater than 97% purity.
The monomers comprising sulfonic acid groups may moreover also be
used in the form of derivatives, which may then be converted into
the acid, the conversion into the acid also possibly proceeding in
the polymerised state. These derivatives include in particular the
salts, esters, amides and halides of the monomers comprising
sulfonic acid groups.
The monomers comprising sulfonic acid groups may moreover also be
introduced onto and into the membrane after hydrolysis. This may
proceed by means of per se known measures (for example spraying,
dipping etc.) which are known from the prior art.
In a further embodiment of the invention, monomers capable of
crosslinking may be used. These monomers may be added to the
hydrolysis liquid. The monomers capable of crosslinking may
furthermore also be applied onto the membrane obtained after
hydrolysis.
The monomers capable of crosslinking are in particular compounds
which comprises at least 2 carbon-carbon double bonds. Preferred
monomers are dienes, trienes, tetraenes, dimethyl acrylates,
trimethyl acrylates, tetramethyl acrylates, diacrylates,
triacrylates, tetraacrylates.
Particularly preferred monomers are dienes, trienes, tetraenes of
the formula
##STR00013## dimethyl acrylates, trimethyl acrylates, tetramethyl
acrylates of the formula
##STR00014## diacrylates, triacrylates, tetraacrylates of the
formula
##STR00015## in which R means a C1-C15 alkyl group, C5-C20 aryl or
heteroaryl group, NR', --SO.sub.2, PR', Si(R).sub.2, wherein the
above residues may in turn be substituted, R' mutually
independently means hydrogen, a C1-C15 alkyl group, C1-C15 alkoxy
group, C5-C20 aryl or heteroaryl group and n is at least 2.
The substituents of the above residue R preferably comprise
halogen, hydroxyl, carboxy, carboxyl, carboxyl ester, nitrile,
amine, silyl, siloxane residues.
Particularly preferred crosslinking agents are allyl methacrylate,
ethylene glycol dimethacrylate, diethylene glycol dimethacrylate,
triethylene glycol dimethacrylate, tetra- and polyethylene glycol
dimethacrylate, 1,3-butanediol dimethacrylate, glycerol
dimethacrylate, diurethane dimethacrylate, trimethylpropane
trimethacrylate, epoxy acrylates, for example Ebacryl,
N',N-methylenebisacrylamide, carbinol, butadiene, isoprene,
chloroprene, divinylbenzene and/or bisphenol-A dimethyl acrylate.
These compounds are commercially obtainable for example from
Sartomer Company Exton, Pa. under the names CN-120, CN.sub.104 and
CN-980.
The use of crosslinking agents is optional, wherein these compounds
may conventionally be used in the range between 0.05 to 30 wt. %,
preferably 0.1 to 20 wt. %, particularly preferably 1 and 10 wt. %,
relative to the weight of the membrane.
The crosslinking monomers may also be applied by spraying etc.
According to one particular aspect of the present invention, the
monomers comprising phosphoric acid and/or sulfonic acid groups or
the crosslinking monomers be polymerised, wherein polymerisation
preferably proceeds free-radically. Free-radical formation may
proceed thermally, photochemically, chemically and/or
electrochemically.
A starter solution which contains at least one substance capable of
forming free radicals may be added to the hydrolysis liquid. A
starter solution may moreover be applied onto the membrane after
hydrolysis. This may proceed by means of per se known measures (for
example spraying, dipping etc.) which are known from the prior
art.
Suitable free-radical formers are inter alia azo compounds, peroxy
compounds, persulfate compounds or azoamidines. Non-limiting
examples are dibenzoyl peroxide, dicumene peroxide, cumene
hydroperoxide, diisopropyl peroxydicarbonate,
bis(4-t-butylcyclohexyl)peroxydicarbonate, dipotassium persulfate,
ammonium peroxydisulfate, 2,2'-azobis(2-methylpropionitrile)
(AlBN), 2,2'-azobis-(isobutyric acid amidine) hydrochloride,
benzopinacole, dibenzyl derivatives, methyl ethylene ketone
peroxide, 1,1-azobiscyclohexanecarbonitrile, methyl ethyl ketone
peroxide, acetylacetone peroxide, dilauryl peroxide, didecanoyl
peroxide, tert.-butyl per-2-ethylhexanoate, ketone peroxide, methyl
isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl
peroxide, tert.-butyl peroxybenzoate, tert.-butylperoxyisopropyl
carbonate, 2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane,
tert.-butylperoxy-2-ethylhexanoate,
tent.-butylperoxy-3,5,5-trimethylhexanoate,
tert.-butylperoxyisobutyrate, tert.-butylperoxyacetate, dicumyl
peroxide, 1,1-bis(tert.-butylperoxy)-cyclohexane,
1,1-bis(tert.-butylperoxy)-3,3,5-trimethylcyclohexane, cumyl
hydroperoxide, tert.-butyl hydroperoxide,
bis(4-tert.-butylcyclohexyl) peroxydicarbonate, and the
free-radical formers obtainable from DuPont under the name
.RTM.Vazo, for example .RTM.Vazo V50 and .RTM.Vazo WS.
It is moreover also possible to use free-radical formers which form
free radicals on irradiation. Preferred compounds include inter
alia .alpha.,.alpha.-diethoxyacetophenone (DEAP, Upjon Corp),
n-butyl benzoin ether (.RTM.Trigonal-14, AKZQ) and
2,2-dimethoxy-2-phenylacetophenone (.RTM.Igacure 651) and
1-benzoylcyclohexanol (.RTM.Igacure 184),
Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide (.RTM.Irgacure
819) and 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-phenyl
propan-1-one (.RTM.Irgacure 2959), which are in each case
commercially obtainable from Ciba Geigy Corp.
Conventionally, between 0.0001 and 5 wt. %, in particular 0.01 to 3
wt. % (relative to the weight of the free-radically polymerisable
monomers; monomers comprising phosphonic acid and/or sulfonic acid
groups or the crosslinking monomers) of free-radical former is
added. The quantity of free-radical former may be varied depending
on desired degree of polymerisation.
Polymerisation may also proceed by exposure to IR or NIR
(IR=infrared, i.e. light with a wavelength of greater than 700 nm;
NIR=near IR, i.e. light with a wavelength in the range from approx.
700 to 2000 nm or with an energy in the range from approx. 0.6 to
1.75 eV).
The polymerisation may also proceed by exposure to UV light with a
wavelength of less than 400 nm. This polymerisation method is per
se known and described, for example, in Hans Joerg Elias,
Makromolekulare Chemie, 5th edition, volume 1, pp. 492-511; D. R.
Arnold, N. C. Baird, J. R. Bolton, J. C. D. Brand, P. W. M Jacobs,
P. de Mayo, W. R. Ware, Photochemistry--An Introduction, Academic
Press, New York and M. K. Mishra, Radical Photopolymerization of
Vinyl Monomers, J. Macromol. Sci.-Revs. Macromol. Chem. Phys.
C22(1982-1983) 409.
Polymerisation may also be achieved by exposure to .beta., .gamma.
and/or electron beam radiation. According one particular embodiment
of the present invention, a membrane is irradiated with a radiation
dose in the range from 1 to 300 kGy, preferably from 3 to 200 kGy
and very particularly preferably from 20 to 100 kGy.
Polymerisation of the monomers comprising phosphonic acid and/or
sulfonic acid groups or of the crosslinking monomers preferably
proceeds at temperatures above room temperature (20.degree. C.) and
less than 200.degree. C., in particular at temperatures between
40.degree. C. and 150.degree. C., particularly preferably between
50.degree. C. and 120.degree. C. Polymerisation preferably proceeds
under normal pressure, but may also proceed with exposure to
pressure. Polymerisation leads to solidification of the planar
structure, it being possible to monitor this solidification by
measurement of microhardness. The increase in hardness brought
about by polymerisation preferably amounts to at least 20%,
relative to the hardness of the planar structure obtained in step
B).
According to one particular aspect of the present invention, the
molar ratio of the molar sum of phosphoric acid, polyphosphoric
acid and the hydrolysis products of polyphosphoric acid to the
number of moles of phosphonic acid groups and/or sulfonic acid
groups in the polymers obtainable by polymerisation of monomers
comprising phosphonic acid groups and/or monomers comprising
sulfonic acid groups is preferably greater than or equal 1:2, in
particular greater than or equal 1:1. and particularly preferably
greater than or equal 2:1.
The molar ratio of the molar sum of phosphoric acid, polyphosphoric
acid and the hydrolysis products of the polyphosphoric acid to the
number of moles of phosphonic acid groups and/or sulfonic acid
groups in the polymers obtainable by polymerisation of monomers
comprising phosphonic acid groups and/or monomers comprising
sulfonic acid groups is in the range from 1000:1 to 3:1, in
particular 100:1 to 5:1 and particularly preferably 50:1 to
10:1.
The molar ratio may be determined using conventional methods.
Spectroscopic methods, for example NMR spectroscopy, may in
particular be used for this purpose. It must be borne in mind that
the phosphonic acid groups are present in the formal oxidation
number 3 and the phosphorus in phosphoric acid, polyphosphoric acid
or the hydrolysis products thereof is present in the oxidation
number 5.
Depending on the desired degree of polymerisation, the planar
structure which is obtained after polymerisation is a
self-supporting membrane. The degree of polymerisation preferably
amounts to at least 2, in particular at least 5, particularly
preferably at least 30 repeat units, in particular at least 50
repeat units, very particularly preferably at least 100 repeat
units. This degree of polymerisation is determined from the
number-average molecular weight M.sub.n, which may be determined by
GPC method. Due to the problems of isolating the polymers
containing phosphonic acid groups which are contained in the
membrane without degradation, this value is determined by making
use of a sample which is prepared by polymerising monomers
comprising phosphonic acid groups without addition of polymer. The
proportion by weight of monomers comprising phosphonic acid groups
and of free-radical starter is here kept constant in comparison
with the ratios for production of the membrane. The degree of
conversion which is achieved in a comparison polymerisation is
preferably greater than or equal 20%, in particular greater than or
equal 40% and particularly preferably greater than or equal 75%,
relative to the introduced monomers comprising phosphonic acid
groups.
The hydrolysis liquid comprises water, the concentration of the
water not generally being particularly critical. According to one
particular aspect of the present invention, the hydrolysis liquid
comprises 5 to 80 wt. %, preferably 8 to 70 wt. % and particularly
preferably 10 to 50 wt. % water. The quantity of water which is
formally present in the oxo acids is not taken into account in the
water content of the hydrolysis liquid.
Of the above-stated acids, phosphoric acid and/or sulfuric acid are
particularly preferred, these acids in particular comprising 5 to
70 wt. %, preferably 10 to 60 wt. % and particularly preferably 15
to 50 wt. % water.
Subsequent to the moisture treatment, the membrane may be further
crosslinked by exposure to heat in the presence of oxygen. This
curing of the membrane additionally improves the properties of the
membrane. The membrane may be heated to a temperature of at least
150.degree. C., preferably at least 200.degree. C. and particularly
preferably at least 250.degree. C. The oxygen concentration in this
method step is conventionally in the range from 5 to 50 vol. %,
preferably 10 to 40 vol. %, without this being intended to
constitute a limitation. This crosslinking too may also proceed by
exposure to IR or NIR (IR=infrared, i.e. light with a wavelength of
greater than 700 nm; NIR=near IR, i.e. light with a wavelength in
the range from approx. 700 to 2000 nm or with an energy in the
range from approx. 0.6 to 1.75 eV). A further method is irradiation
with .beta. radiation. The radiation dose here amounts to between 5
and 200 kGy.
Depending on the desired degree of crosslinking, the duration of
the crosslinking reaction may vary widely. In general, this
reaction time is in the range from 1 second to 10 hours, preferably
1 minute to 1 hour, without this being intended to constitute a
limitation.
The method according to the invention permits comparatively simple
and inexpensive production of acid-doped, polyazole-containing
membranes, which method can readily be scaled up to a large
industrial scale. The following advantages may in particular be
achieved by the approach according to the invention: distinctly
less, usually no, solvent is required for production of the
membranes, production of the membranes may proceed with a
distinctly better space-time yield, membranes with comparatively
high quality and reproducibility are obtained with virtually no
fluctuations in quality between different batches being observed,
it is now possible to process polyazoles with comparatively high
molecular weights and the formation of bubbles in the membrane is
virtually completely prevented.
Fields of application of the membranes obtainable by the method
according to the invention in particular include the use thereof as
a polymer electrolyte membrane in fuel cells. Further details may
be found by referring to documents DE 102 13 540 A1, DE 102 46 559
A1 and DE 102 46 461 A1, the disclosure of which is hereby
incorporated by reference.
* * * * *